This invention relates to a paper size composition using a selected modified cationic starch and to a method of sizing paper and paperboard therewith. More particularly, this invention relates to a paper size composition comprising a mixture of a hydrophobic sizing agent and a selected modified cationic starch.
The use of sizing in paper is known to provide several beneficial attributes to the paper and the processing thereof including paper strength, retarding liquid penetration into the sheet and the quality, suitability and ease of printing on the paper.
Paper and paperboard are often sized with various hydrophobic materials including, for example, rosin, wax emulsions, mixtures of rosin waxes, ketene dimers, isocyanate derivatives, fatty acid complexes, fluorocarbons, certain styrene-maleic anhydride copolymers as well as the substituted cyclic dicarboxylic acid anhydrides, more particularly described hereinafter. These sizes are introduced during the actual paper making operation and, as such, require that the sizing compounds be uniformly dispersed throughout the fiber slurry in a small particle size. General practice therefore, has been to add the sizes in the form of an aqueous emulsion prepared with the aid of emulsifying agents such as cationic or ordinary starches, carboxymethyl cellulose, natural gums, gelatin, cationic polymers or polyvinyl alcohol, all of which act as protective colloids. The use of such emulsifying agents, with or without added surfactants, does, however, suffer from several inherent deficiencies in commercial practice. A primary deficiency concerns the necessity of utilizing relatively complex, expensive and heavy equipment capable of exerting high homogenizing shear and/or pressures, together with demanding procedures regarding emulsifying proportions and temperatures, etc., for producing a satisfactory stable emulsion of the particular size. Additionally, the use of many surfactants in conjunction with protective colloids is found to create operational problems in the paper making process such as severe foaming of the stock and/or loss in sizing.
With particular reference to the procedure of the prior art which utilizes substituted cyclic dicarboxylic acid anhydrides as sizing agents, it has been necessary in commercial practice to pre-emulsify with cationic starch and/or hydrocolloids using relatively demanding procedures with elevated temperatures to cook the starch or hydrocolloids and high shearing and/or high pressure homogenizing equipment. Unless these complicated procedures are carefully followed, difficulties such as deposition in the paper system, quality control problems and generally unsatisfactory performance are often encountered.
Many of these problems have been overcome by the teachings of U.S. Pat. No. 4,214,948 and U.S. Pat. No. Re 29,960 which disclose the use of a size mixture of specific sizing agents and polyoxyalkylene alkyl or alkyl-aryl ethers or their corresponding mono- or di-esters, which mixtures are easily emulsifiable with water in the absence of high shearing forces and under normal pressure.
Other useful size mixtures include the disclosure of a composition of a hydrophobic substituted cyclic dicarboxylic acid anhydride and an ethoxylated lanolin, as shown in U.S. Pat. No. 4,747,910, or ethoxylated castor oil shown in U.S. Pat. No. 4,832,792. Japanese Patent Disclosure Bulletin 60-88196, dated May 17, 1985 discloses dispersion stable sizing compositions comprising ketene dimers and a highly degraded hydroxyalkyl modified cationic starch. Another Japanese patent Disclosure Bulletin 58-197397, dated Nov. 17, 1983 discloses the preparation of sizing agents using selected cationic starch derivatives having a defined low inorganic ion concentration in combination with neutral sizing agents. Ether and ester modifications are provided along with significant molecular weight reduction by starch degradation in order to provide suitable emulsion stability. Another useful paper size composition and method is disclosed in U.S. Pat. No. 4,721,655 wherein a sizing emulsion comprising a hydrophobic sizing agent and a jet cooked dispersion of a long chain hydrophobic starch or gum derivative is shown.
Despite the contributions of the above noted patents and disclosures there remains a need in the art for sizing emulsions exhibiting improved emulsion quality, sizing performance and operability.